Actually, the Republicans love Ocasio-Cortez, in the same way Democrats love Mark Meadows and others among the Republicans' far-right Freedom Caucus. They hate her politics, but they hope the young representative will sow division among Democrats. They were booing her because, this time, she didn't.

The decision by Ocasio-Cortez and others on the far left about whether to work with or against their party will determine the fate of the new majority and of the resurgent progressive movement. The Democrats' return to power after wandering in the wilderness for eight years — and 20 of the past 24 — holds both great promise and great peril for them.

If they can stay unified, they will be an effective counterweight to the Trump lunacy, establishing the Democrats as the party to be entrusted with governing. But if they are split by internal divisions, they could become an easy foil for President Trump, lose suburban seats that gave them the House majority, and possibly hand Trump a second term.

The country is on fire. This is the time for Democrats to be the grown-ups voters want.

It's not the time for Rep. Brad Sherman, D-Calif., without waiting for the Mueller report, to announce plans to introduce articles of impeachment against Trump.

Progressives aren't solely to blame. Even after a rebellion by moderates got Pelosi to accept leadership term limits, 15 Democrats refused to vote for her on the floor — including freshman Rep. Jeff Van Drew (N.J.).

"No," he called out when the clerk asked who he was voting for.

Dr. No?

Because "No" is not a person. Van Drew's vote — dissent for the sake of dissent — was switched to "present."

At the fulcrum between Democratic unity and division is Ocasio-Cortez, a social media sensation who has endured hysterical attacks from the right.

A few hours after her vote for Pelosi, Ocasio-Cortez swung the other way on the first substantive vote: She opposed a resolution setting out new House rules, painstakingly negotiated by the entire Democratic caucus.

Her objection: a bit of accounting arcana known as "paygo." She accused her Democratic colleagues of "a dark political maneuver designed to hamstring progress" on health care and other legislation. The passionate dissent was curious, given that the proposed rule is already current law, was a significant improvement over the Republican rule and, anyway, is routinely disregarded. Only two Democrats joined her rebellion.

Ocasio-Cortez has become known for such stands. She joined a sit-in at Pelosi's office demanding a select committee on climate change — though Pelosi had said two weeks earlier that she favored such a committee.

Later, Politico reported that Ocasio-Cortez was seeking a 2020 primary challenger to Rep. Hakeem Jeffries, D-N.Y., a popular African American progressive and House leader. She denied it.

She and other left-wing newcomers can have a salutary effect. Their protest over lobbyists' presence at an unofficial orientation at Harvard for new members led to a rethinking of the event. Their advocacy for Medicare-for-all health coverage has nudged Pelosi to accept hearings.

But now comes decision time. Will Ocasio-Cortez and fellow hard-liners become the left's version of the Freedom Caucus? Will they object to H.R. 1, the Democrats' ethics and voting-rights package, because it doesn't go far enough in banning corporate money? Will they withhold support for bills unless they can force votes on Medicare-for-all and abolishing Immigration and Customs Enforcement? Or will the firebrands build support for their causes without forcing vulnerable colleagues to cast suicidal votes on bills that won't become law?

Democratic unity is what gives them the upper hand in the shutdown battle, as some Republicans openly question Trump's strategy. Democratic unity also allows them to appeal to the large majority of Americans disgusted with Trump, as Pelosi did during her acceptance speech, uttering "bipartisan" seven times, praising George H.W. Bush and approvingly quoting Ronald Reagan on immigration.

There was silence on the Republican side, now a shrunken sea of old white men. "You don't applaud for Ronald Reagan?," Pelosi taunted.

A disastrous presidency has given members of the progressive movement an extraordinary opportunity — if they don't blow it by fighting among themselves.

Dana Milbank is an op-ed columnist. He sketches the foolish, the fallacious and the felonious in politics. Follow him on Twitter, @Milbank.

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